The present invention relates to a component comprising an element, which comprises zirconium or a zirconium alloy and which has a surface on which a corrosion protective oxide layer is formed, which oxide layer comprises zirconium oxide, wherein the component is intended to be in an oxidising environment and said oxide layer has an outer surface towards said oxidising environment. The invention also relates to a method for manufacturing a component comprising an element, which comprises zirconium or a zirconium alloy and which has a surface on which a corrosion protective oxide layer is formed, which comprises zirconium oxide, wherein the component intended to be in an oxidising environment and said oxide layer has an outer surface towards said oxidising environment, and a nuclear facility comprising a reactor.
In a nuclear facility, the reactor core comprises a number of fuel assemblies, which comprise a top plate and a bottom plate with a number of elongated, parallely placed tubular elements extending between the top plate and the bottom plate and a number of spacers axially distributed along and connected to the elongated tubular elements. The fuel assembly also comprises a large number of elongated fuel rods which are kept in place by the spacers between the top plate and the bottom plate according to a specific pattern and on specific distances relative to each other and to the elongated tubular elements. Each fuel rod contains nuclear fuel enclosed in a cladding tube. When the nuclear facility is in operation the reactor core is cooled with the help of a cooling medium being pumped upwards through the reactor core.
Components in nuclear facilities are often subjected to attacks caused by hydration and oxidisation hence it may be interesting to deposit a coating on the surface of the components to protect them. Cladding tubes for nuclear fuel is an example of such components. An attack on a cladding tube for nuclear fuel means in the worst possible case that a damage extending through the entire thickness of the cladding tube occurs so that the radioactive nuclear fuel inside the cladding tube leaks out into the cooling water of the reactor. This may be caused by both primary and secondary defects on the cladding tube.
Primary defects occur from attacks on the outer surface of the cladding tube and are in particular caused by wear from foreign objects. A primary defect extending through the whole thickness of the cladding tube means that water, steam or a combination of these streams in through the defect so that an area between the fuel and the inner surface of the cladding tube is filled with the water, the steam or the combination of these. The presence of the water, the steam or the combination of these in this area means that the cladding tube is in danger of being damaged through attacks on the inside of the tube. This attack is often caused through hydration. Such a defect is called a secondary defect and can only occur after a primary defect already has occurred. Both primary defects and secondary defects extending through the entire thickness of the cladding tube means that the nuclear fuel inside the cladding tube, and thus radioactivity, leaks out into the cooling water of the reactor. Secondary defects may occur at relatively long distances from the primary defects and have also usually the shape of long cracks or transverse breaks meaning that they are a serious type of defect.
Zirconium is often used in materials for cladding tubes, which enclose the nuclear fuel in reactors because of its low neutron absorption ability and its good corrosion resistance. In order to enhance the mechanical properties of the material and to increase its corrosion resistance the cladding material is alloyed for example with tin with about a percentage of weight and with smaller contents of iron, nickel and chrome. The total content of alloying materials can be about some percentages of weight. During operation the cooling water reacts with the cladding tube and forms zirconium oxide, wherein hydrogen is released. Zirconium oxide is relatively corrosion resistant. In an oxidising environment this depends on the fact that the. oxide layer formed can resist further attacks. How efficiently the oxide layer can resist further attacks is mostly governed by the oxygen and hydrogen transport in the oxide. Hydrogen can react with the cladding material and form zirconium hydride, which can lead to large cracks being formed in the cladding material. It should be noted, however, that small amounts of contaminants might exist in the alloy, wherein hydrogen is usually mentioned. A typical content of hydrogen is about 10 ppm by weight.
That hydrogen is regarded as a contaminant is based partly on the knowledge of embrittlement of a metal through absorption of large amounts of hydrogen, a so-called hydrogen embrittlement. Hydrogen embrittlement means that the metal cracks more easily during mechanical stress. Hydrogen embrittlement may happen if the metal is heated in a hydrogen rich environment or if hydrogen is formed at the metal surface, for example in connection with corrosion, pickling and electrochemical surface treatment.
How efficiently the oxide layer can resist further attacks also depends on the presence of microscopic pores, cavities and cracks in the oxide layer. An oxide layer that only grows in the interface between the oxide layer and the surrounding gas/liquid by way of metal transport through the oxide layer tends to have a bad adherence to the metal substrate. Furthermore an oxide layer growing only in the interface between the metal substrate and the oxide layer tends to crack in the interface towards the surrounding gas/liquid. A prerequisite for pores, cavities and cracks to be able to disappear during the growth of the oxide layer is that an oxide is formed at the surfaces adjacent to these pores, cavities and cracks which in turn assumes that oxygen, preferably in dissociated form, and metal are transported to said surfaces.
In “Oxidation of Metals, Vol. 51, Nos ¾, 1999; Hydrogen in Chromium: Influence on the high-temperature Oxidation Kinetics in O2, Oxide-Growth Mechanisms, and Scale Adherence; B Tveten et al.” it has been reported that an increased hydrogen content in chromium leads to increased metal transport and increased oxide growth in the interface between the oxide layer and the surrounding gas phase. It is also reported in said article that the presence of hydrogen in the metal results in less adherence of the oxide layer to the metal through decreased oxidisation in the interface between the metal and the oxide layer. Thus the presence of hydrogen in metal contributes to a decreased corrosion resistance.